The present invention relates generally to methods and apparatuses for performing detection of synchronization failure, and more particularly to a method and apparatus for performing detection of synchronization failure in an Asynchronous Transfer Mode (ATM) network.
Within an ATM network, the ATM Adaptation Layer (AAL) is responsible for performing conversions from non-ATM traffic to ATM traffic, and vice versa That is, signal traffic that enters the ATM network in non-packetized form must be converted into packets, each which are transmitted separately over the ATM network and then the original signal is reconstructed at the terminal of the ATM network. The ATM Adaptation Layer type 1 (AAL1) performs this conversion for Constant-Bit-Rate (CBR) signals. Private line service traffic is an excellent example of a CBR signal. A critical function of the AALl protocol is the recovery of the original frequency of the CBR signal at the egress node of the ATM network. The signals enter the ATM network at some nominal frequency, e.g., approximately 1.544 Megabits per second (Mb/s) for DS1s. The signals must be output from the ATM network at the same nominal frequency.
There are several methods for recovering the nominal frequency at the egress node of the network. Two of the most commonly employed techniques are the Synchronous Residual Time Stamp (SRTS) method, and the adaptive method.
Among the various frequency recovery schemes that have been implemented, the Synchronous Residual Time Stamp (SRTS) scheme has several advantages and it is implemented in ATM switches of many vendors. Furthermore, SRTS is the only frequency recovery scheme supported by the American National Standards Institute (ANSI). As a result, SRTS is the preferred frequency recovery scheme of most vendors.
The other frequency recovery method used in this application is the so-called adaptive method. The adaptive method recovers the original frequency from the cells (i.e., packets) as they are output from the network to the terminal node. By monitoring the rate at which the cells exit the ATM network, the adaptive method recovers the original frequency of the CBR signal, similar to a phase lock loop that recovers the frequency of the signal from the incoming data. Because the adaptive method assumes that the ATM network has not added variations that are significant relative to the frequency of the CBR signal, the resulting recovered frequency is not always accurate, especially during congested periods, during which time the ATM network can add relatively significant delay variation to the packets, which is known as Cell Delay Variation (CDV). Consequently, the rate of the outgoing cells may be significantly different than the rate of the incoming cells due to delays in the network. As a result, the adaptive method is susceptible to CDV, while SRTS is immune to it as long as the play-out buffer is large enough to absorb the CDV.
In order to use the SRTS frequency recovery scheme, however, a common clock has to be available at both the ingress and egress nodes of the ATM network. This common clock is used to encode the frequency of the CBR signal at the ingress node on to the AAL 1 overhead byte carried by cells and to reconstruct the frequency of the CBR signal at the egress node of the ATM network. When the common network clock is lost due to synchronization failures at either the ingress or egress ATM node, the reconstructed frequency of the CBR signal will be different from the original frequency of the CBR signal. Such a difference will cause buffers to underflow/overflow (slip) and will result in loss of data.
The present invention is therefore directed to the problem of developing a method and apparatus for automatically detecting a synchronization failure in an ATM network.